1. Field of the Invention
The present invention relates to a yoke of a planetary gear-type starter incorporating a planetary gear reducer used in, for example, an automobile engine, a manufacturing apparatus therefor and a manufacturing method thereof, and more specifically, to the yoke of a planetary gear-type starter in which the reduction in weight of the yoke can be easily realized, as well as a manufacturing apparatus therefor and a manufacturing method thereof.
2. Description of the Related Art
Conventionally, a planetary gear-type starter incorporates a planetary gear reducer which includes planetary gears as a starter for driving, for example, a crank shaft when an automobile engine is started.
FIG. 7 is a side elevational view, partly in cross section, of a planetary gear-type starter including a conventional yoke arranged integrally with an inner peripheral gear portion.
In FIG. 7, the yoke 1 acting as a field magnet of the motor of the planetary gear-type starter includes an annular inner peripheral gear portion 11 formed to an inner peripheral side adjacent to a bottom portion 12 composed of a thick portion, a bearing 13 engaged in the opening 12a of the bottom portion 12 and a cylindrical thin portion 14 formed to an opening side confronting the bottom portion 12.
A flange 2 journaled by the bearing 13 has a plurality of pins 22 standing on the end surface extending in the yoke 1. The respective pins 22 are concentrically disposed to form an arc shape with respect to the center axis of rotation of the flange 2 and each of the pins 22 has a planetary gear 4 rotatably disposed thereto.
The flange 2 constitutes a reducer on the end surface side thereof in cooperation with the inner peripheral gear portion 11 and planetary gears 4. The other end of the flange 2 is fixed to the starter output shaft 21 of the starter so as to transmit the rotational output of a motor output shaft 31 to the starter output shaft 21 through the planetary gear reducer.
An annular projecting portion 12b projecting in the axial direction of the starter output shaft 21 is formed to the end surface of the opening 12a of the bottom portion 12 so as to secure the length in the axial direction so that the projecting portion holds the bearing 13 by being engaged therewith. With this arrangement, even if the bottom portion 12 is formed to a relatively thin thickness, it is permitted to be engaged with the bearing 13, so that reduction in weight is realized by suppressing the wall thickness of the bottom portion 12.
The extreme end of the starter output shaft 21 is journaled by a bracket 5 covering a front portion, and an overrunning clutch 24 and a pinion gear 25 are disposed at an intermediate portion of the starter output shaft 21 so as to be movable in the axial direction. The bracket 5 is fixed to the outer peripheral end surface of the bottom portion 12 of the yoke 1.
When the starter is operated, the overrunning clutch 24 and the pinion gear 25 move in the direction of the arrow while rotating integrally with the starter output shaft 21, so that they drive a starting object (for example, the crank shaft of an automobile engine) through the gear portion of the outer peripheral side of the pinion gear 25.
The motor which is driven at the time of start includes a rotor or an armature 3 coupled with the motor output shaft 31, a permanent magnet 33 acting as a field magnet device and the yoke 1. The permanent magnet 33 is magnetized in a radial direction and generates flux in the peripheral direction in the yoke 1 acting as a magnetic path forming unit. The motor output shaft 31 coupled with the armature 3 is accommodated in the thin portion 14 of the cylindrical yoke 1. The extreme end of the motor output shaft 31 extends to the vicinity of the bearing 13 in the yoke 1.
The motor output shaft 31 is coaxially accommodated in the flange 2 of the starter output shaft 21 and journaled by the bearing 23 in the flange 2.
The plurality of planetary gears 4 rotatably mounted to the respective pins 22 on the flange 2 are engaged with the outer peripheral gear portion 32 on the motor output shaft 31 and the inner peripheral gear portion 11 of the yoke 1, respectively.
The inner peripheral gear portion 11 of the yoke 1 constitutes the planetary gear reducer in association with the planetary gears 4 on the flange 2 and the motor output shaft 31 engaging each other, and the planetary gear reducer is accommodated on the bottom portion 12 side of the yoke 1. When the starter is operated, the planetary gear reducer reduces the rotation of the motor output shaft 31 through the planetary gears 4 and transmits the reduced rotation to the starting object (crank shaft) through the starter output shaft 21 and the pinion gear 25.
Although not shown in detail, a switching device 6 which is mounted to the bracket 5 and disposed on the yoke 1 incorporates a switching circuit for supplying electric power to the motor in association with a key switch and the like when the starter is operated and a plunger for driving the overrunning clutch 24.
A lever 7, which is rotatably mounted to a rotatable shaft 71 and has one end engaged with the plunger in the switching device 6 and the other end engaged with the overrunning clutch 24, is rotated counterclockwise about the rotatable shaft 71 when the plunger is driven so as to move the overrunning clutch 24 and the pinion gear 25 on the starter output shaft 21 in the direction of the arrow.
Next, operation of the planetary gear-type starter using the conventional yoke 1 shown in FIG. 7 will be described.
First, before the starter is operated, the lever 7 is located in the state shown in FIG. 7, the overrunning clutch 24 is not moved and the pinion gear 25 is not engaged with the crank shaft.
When the starter is operated by turning on the key switch, the armature 3 of the motor which is energized by the electric power supplied from the switching device 6 is rotated by receiving the urging force of the permanent magnet 33 to thereby drive the motor output shaft 31 arranged integrally with the armature 3 in rotation. In addition, the lever 7 which is driven by the plunger in the switching device 6 rotates counterclockwise about the rotatable shaft 71 so as to move the overrunning clutch 24 in the direction of the arrow and causes the pinion gear 25 to be engaged with the crank shaft.
At the same time, the rotational output of the motor is transmitted to the planetary gears 4 from the outer peripheral gear portion 32 of the motor output shaft 31, so that the planetary gears 4 are rotated as well as being turned between the outer peripheral gear portion 32 of the motor output shaft 31 and the inner peripheral gear portion 11 of the yoke 1.
With this operation, the flange 2 to which the planetary gears 4 are rotatably mounted is rotated at a rotational speed which is reduced with respect to the rotational speed of the motor output shaft 31 and the reduced rotational output is transmitted to the starter output shaft 21.
The starter output shaft 21 rotates the crank shaft at the reduced rotational speed through the overrunning clutch 24 and the pinion gear 25.
The manufacturing apparatus and the manufacturing method of the conventional yoke 1 will be described with reference to FIG. 8 and FIG. 9.
FIG. 8 is a side cross sectional view showing a yoke 1 formed using an ordinary cold forging apparatus, and FIG. 9 is a front elevational view showing the state that a bearing 13 is removed from the yoke 1 of FIG. 8 which is viewed from a thin portion 14 side, wherein components denoted by the same numerals are the same as those in the above-mentioned figure (see FIG. 7).
FIG. 8 shows a case that an annular projecting portion 12b is not formed to the bottom portion 12 of the yoke 1 because the bottom portion 12 has a wall thickness sufficient to permit the disposition of the bearing 13.
The structure of FIG. 8 can be more easily made than the structure of FIG. 7 in which the annular projecting portion 12b is formed. This is because the yoke 1 is conventionally made by cold forging, sintering, sintering forging, injection molding, powder metallurgy and the like, and the formation of the annular projecting portion 12b as shown in FIG. 7 increases the number of steps of a manufacturing process.
Many working steps are needed to form the thin portion 14 which is much thinner than the bottom portion 12 by working a yoke material originally having the same wall thickness. In addition, since a degree of working (corresponding to the reduction ratio of a cross section) for forming the thin portion 14 is increased, when cold forging or the like is used in particular, a large amount of pressurizing force is required, by which the life of a tool is deteriorated and the cost is increased.
Further, when sintering and the like is used, it is difficult to fill the thin portion 14 with a predetermined amount of material.
As described above, the conventional yoke of the planetary gear-type starter as well as the manufacturing apparatus therefor and the manufacturing method thereof have problems in that since the yoke is molded using cold forging, sintering and the like, the bottom portion 12 composed of the thick portion and the cylindrical thin portion 14 cannot be easily formed, miniaturization and reduction in weight are not easily realized and a large manufacturing cost is needed to form the thin portion 14.
An object of the present invention made to solve the above problems is to provide a yoke of a planetary gear-type starter in which an inner peripheral gear portion and a thin portion are easily formed and miniaturization, reduction in weight and cost reduction are realized by using cold spinning working, as well as providing a manufacturing apparatus therefor and a manufacturing method thereof.
Another object of the present invention is to provide a yoke of a planetary gear-type starter in which an annular projecting portion is easily formed from an opening by burring working when a disk serving as the metal material of the yoke is mounted to a working tool and miniaturization, reduction in weight and cost are realized in cold spinning working, as well as providing the manufacturing apparatus therefor and the manufacturing method thereof.